What is in this article?:

Users of hydraulic systems are demanding cost effective, leak-proof and reliable components with high power density. This means higher working pressures, increased simplification of piping systems, and fewer potential leak points. A limitation for increasing the pressure capacity of existing threaded port systems is that the seal diameter is the thread undercut diameter. To accommodate higher-pressures, the fitting body cross section area must be increased. But this would also increases the seal diameter and pressure acting area, resulting in a higher load force and no gain in pressure holding capacity.

In the new SAE J2337 port and fitting system, sealing occurs in the bottom of the port, which, on average, doubles pressure capacity. Moreover, the swivel nut feature eliminates the need for adapter fittings and, therefore, can cut the number of potential leak points in half. Test results support working pressures up to 80,000 with a design factor of 4:1.

Demands from industry

A priority for the 21st century among users of hydraulic systems is to maintain or reduce component cost. The best way to reduce cost is to increase component power density - that is, the ratio of power output to the weight or volume of the component. The only way to increase or maintain the power output with a simultaneous decrease of volume and component weight is to increase the system working pressure.

Higher performance - In some industries, such as aerospace, the size and efficiency of components - not only cost - are critical factors. The continuing trend is for lighter components with higher performance - known as miniaturization. Miniaturization of parts can be accomplished only if the system pressure is increased. To meet requirements of the aerospace industry, system pressures of up to 8000 psi are not uncommon.

Leak-free systems - A potential problem that can accompany higher operating pressure is leakage. Forty years ago, hydraulic systems typically operated at 300 psi. Today, 3000-psi systems are common, and some are rated at 5000 or even 6000 psi.

Another potential problem that can accompany high-pressure systems is pressure spikes and high cycling of dynamic loads that can cause unpredictable peak pressures. Consequently, fittings and hoses are subjected to high stress. Therefore, to prevent leakage, fittings must contain fluid at these higher pressures.

Current limitations

Tapered threads - Although the tapered pipe thread (NPT) is still in use, several of its major characteristics significantly reduce its effectiveness. The dry seal thread requires use of sealing tape or a sealing compound to prevent leakage. Furthermore, they increase assembly costs and threaten to contaminate working fluids whose cleanliness is crucial for reliable operation.

Swelling, deformation, internal stresses, and cracking of the component body may result because of the conical nature and the extremely high radial forces created when tightening the fitting body into the port. Furthermore, in some valve and pump designs, even low stresses can deform adjacent passageways, such as the bore for valve spools and closely machined ports. To compensate for this limitation, components must have a large bulky design, which results in excess weight and higher costs.

Orientation problems with the tapered thread joint have always been a big disadvantage. Aprroximately two turns are needed after hand tight engagement to get proper sealing. But at the same time, the fitting often is oriented in the wrong direction. Assemblers often encounter installation problems that result in an unreliable joint and increased high costs.

O-Ring boss port fittings - Straight thread O-ring boss port fittings, Figure 2 (SAE 2244, ISO 6149, DIN 3852, JIS B 2351) were developed in the early 1950s and are a good solution for many hydraulic systems. However, considering today's higher system pressures, it is at its upper performance limit based on its design.

The pressure rating for a straight connector is 6000 psi for smaller sizes and 4000 psi for larger sizes. For adjustable fittings, the pressure rating is 6000 psi for smaller sizes and 3000 psi for larger sizes. SAE J2244 is rated to 9100 psi.

Furthermore, the potential for any improvements is limited. Assuming that the use of stronger heat-treated material is unacceptable because of cost, and that flow capacity cannot be reduced, the following explanation shows the limitations:

If a stronger fitting is needed, stress from fluid pressure must be reduced. To lower the stress, we must increase area by an increasing the outer diameter. To avoid changing flow capacity, the inner diameter must remain the same. But a substantial increase in fitting body size produces only a slight increase in performance. for example, increasing the OD of the fitting by 10% reduces stress by only 8.3%. Likewise, increasing the OD by 20% or 30% reduces stress by only 14.0% or 19.5%, respectively.

The problem is the pressure load and stress pattern of O-ring boss port fittings. It creates axial load, which tends to pull the connector out from the port; and radial load, which tends to open the female port from pressure acting all the way up to the O-ring itself. The stress pattern of the port is the reason that - even with moderate fluid pressures - requires significant material wall thickness to withstand intensive radial port deformation.

This characteristic especially affects aluminum, cast iron, and weaker materials. Even with a significant increase of wall thickness, fatigue working conditions eventually will produce cracks and failures as a direct consequence of stress distribution. Experimental fatigue testing shows that failure of aluminum housings occurs at a relatively low number of cycles with a 4000-psi peak cycle load applied at only 28 Hz.

Future requirements - Based on international demands of the fluid power industry, fittings for the 21st century must :

* be leak proof at pressures higher than 9000 psi and with a minimum design factor of 4:1, * reduce the number of potential leak points, * be cost effective, * be easy to install, * have a design that will not impose limits on future performance requirements, * be lightweight, * have metric threads, * be non-interchangeable with existing port styles, * be able to accommodate difficult assembly challenges, * not require brazed or welded joints, * be readily reusable, * be able to tolerate overtorquing, * accommodate high flow capacity, and * not exhibit sudden flow passage cross sectional changes, thereby reducing turbulence, heating, and energy loss.